Multi-engine aircraft are designed to maintain flight qualities even when an engine fails. The failure of an engine produces an asymmetrical situation because one side or wing of the aircraft is propelled while the other side or wing of the aircraft is not propelled. This results in yaw, destabilizing the aircraft from a straightahead course.
Nowadays, most aircraft use rudders to compensate for thrust asymmetry. Most of those aircraft have rudder systems and vertical tails sized for this event of single engine failure.
In the case of V/STOL, there is no effective solution because at low speeds there is no rudder authority. Therefore, a different solution is needed to prevent the V/STOL aircraft from yawing upon engine failure.